Growth performance and biochemical composition of Ruditapes decussatus (L.) spat fed on microalgal and wheatgerm flour diets

Abstract

Trials were carried out for different growth diets, consisting wholly or partially of wheatgerm flour, for the spat culture of the little-neck clam, Ruditapes decussatus (L.). The following diets were tested: diet A, consisting of 100% of the daily food ration of the microalga Isochrysis galbana, clone T-ISO; diet B, consisting of 50% microalgae and 50% wheatgerm; diet C, consisting of 25% microalgae and 75% wheatgerm; and diet D, consisting of 100% wheatgerm. Diet A was taken as the reference diet and two control diets were added: suboptimal algal diet B, consisting of 50% of the daily ration of microalgae, and suboptimal algal diet C, consisting of 25% microalgae. Spat growth was determined by live weight, dry weight, organic content and length. Biochemical analyses of the diets and the spat were carried out at the beginning and the end of the 35-day experimental period. The growth achieved when 50% of the daily ration of microalgae was replaced by wheatgerm was similar to that obtained with control diet A, consisting of 100% microalgae. As regards organic matter, the growth of the spat fed on this diet, diet B, was double that shown by the spat fed on suboptimal algal diet B. When 75% of the microalgae were replaced by wheatgerm the growth rates decreased, being only equivalent to 60% of those for reference diet A. Nevertheless, the percentage of organic matter of this spat, indicative of the index of condition, was comparable to that of the spat fed on the reference diet. The total replacement of microalgae by wheatgerm produced a considerable decrease in spat growth, 20% of that obtained with reference diet A. The percentage of organic matter in the spat fed on the extreme diets, diet D and suboptimal diets B and C, was observed to be significantly less than that found in the spat fed on the other diets. Another characteristic of the diets tested was a decrease in protein and lipid content, together with an increase in that of carbohydrates, which corresponded to the extent to which microalgae were replaced by wheatgerm. The biochemical composition of the spat reflects a limitation in the lipid content of diets C and D, reflected in a lower lipid growth rate in the spat compared to that of other body matter.

Introduction

The development of bivalve aquaculture would be greatly enhanced by the total, or even partial, replacement of live algal food by a cheap, easily handled food that has the same nutritive qualities as live food. In bivalve hatcheries, the feeding of spat, i.e., from metamorphosis until it reaches a length of 5–10 mm, accounts for the highest consumption of microalgae (Helm, 1990, Coutteau and Sorgeloos, 1992). It is at this stage of culturing that an alternative food source is therefore most needed.

A suitable alternative diet to that of live food for bivalves must meet the following requirements: particle size suitable for the filtration mechanism of these animals, stability within the culture system, digestibility, null toxicity and a biochemical composition that covers the nutritional needs of each species (Laing, 1989). In accordance with these criteria, and with greater or lesser success, experiments have been conducted with heterotrophically cultivated dry microalgae (Laing et al., 1990, Laing and Verdugo, 1991, Laing and Millican, 1991, Laing and Millican, 1992, Curatolo et al., 1993), yeasts (Epifanio, 1979a, Urban and Langdon, 1984, Nell, 1985), modified yeasts (Albentosa et al., 1989, Coutteau et al., 1991, Coutteau, 1992, Coutteau et al., 1994), microcapsules of different kinds (Gabbott et al., 1976, Langdon and Waldock, 1981, Jones et al., 1984, Langdon and Bolton, 1984, Langdon et al., 1985, Laing, 1987) or bacterial proteins (Nell and Wisely, 1983, Nell and Wisely, 1984, Doulliet and Langdon, 1993, Doulliet and Langdon, 1994). In spite of the large number of studies that have been carried out, no substitute has as yet been found for the totality of the live diet, although substitutions of 40–50% of the live diet have been described.

Another product used in alternative diets for bivalves has been cereal flours, which were first introduced in experiments in the 1960s with the American oyster Crassostrea virginica. In these initial studies (Haven, 1965, Gillespie et al., 1966, Ingle, 1967, Dunathan et al., 1969), flours were used to complement the natural food present in the untreated sea water where adult oysters were kept, in order to stimulate their sexual maturity. Cereal flours have also been used as ingredients of artificial diets when determining the nutritional requirements of different bivalve species (Castell and Trider, 1974, Trider and Castell, 1980, Langdon, 1983, Langdon and Siegfried, 1984, Urban and Langdon, 1984). All of these studies were carried out with adult specimens at the stage of sexual maturity or with juvenile specimens of much greater size than the spat used in our experiment. Other studies performed by our working group with spat as small as that used in the present experiment (Pérez-Camacho et al., 1998) have successfully replaced up to 50% of the daily food ration with cornflour.

In this study, wheatgerm flour is used as a substitute for live microalgae in differing proportions in growth diets for the spat of the little-neck clam, Ruditapes decussatus (L.). The microalga used is Isochrysis galbana, clone T-ISO. We compared biochemical composition of diets with those of R. decussatus and assessed performance of the diets based upon clam growth rates.